Many situations require the detection of power in microwave communication links such as point-to-point communication links for high speed data communications. These wide band communication links run at high gigahertz frequencies, typically up to 24 GHz, and incur the need to measure carrier power in both transmitters and receivers over a wide dynamic range. In fact, the determination of power is indirect. Practically all integrated circuit (IC) solutions used in power measurement applications actually measure the voltage arising across a known load impedance, usually, but not necessarily, 50Ω. For example, a signal of amplitude 223.6 mVrms develops a power of P=V2/R=0.22362/50=0.001 W. RF systems universally express power levels in milliwatts, and use the decibel equivalent expressed in dBm. The power developed by a 223.6 mVrms signal into a 50Ω load is 1 mW, which in decibel form is 0 dBm.
This power level is roughly at the center of typical ranges which need to be measured, ranging roughly from −30 dBm to +13 dBm. In voltage terms, this requires the determination of voltages spanning the range 7.07 mVrms to 1 Vrms, or about 141:1 in voltage terms and 43 dB in power terms.
In the past, RF power measurement has often employed logarithmic amplifiers (log amps) to address the dynamic range challenge. However, at microwave frequencies, suitable log amps require the use of the fastest available IC processes. They are also relatively complicated circuits, needing voltage references to establish their scaling rules, slope and intercept, and consume a great deal of supply current, typically on the order of 100 mA.
On the other hand, detectors based on Schottky diodes inherently have a rapid response, and have been used for decades in power measurement applications. They are now available in many IC processes. Their appeal lies in the fact that, once the RF amplitude is determined (usually referred to as the ‘envelope response’) the remaining signal processing can be carried out at relatively low frequencies, and typically at less than 1/100-th the supply current needed for a log amp. Further, the detection response can readily extend to signal frequencies of 100 GHz, roughly four times higher than the fastest available log amps.
Unfortunately, conventional diode detectors have an extremely nonlinear response. This is especially troublesome at small signal amplitudes; their output at low levels follows a roughly square-law characteristic, and, consequently, they are very insensitive to small signals. As usually implemented, they also have inherently poor temperature behavior, which erodes accuracy. These limitations are overcome by the measures described herein.